1. Field of the Invention
The present invention relates to a power supply circuit, which is capable of restraining generation of power supply noise and fluctuations in a power supply voltage by compensating for current fluctuations caused by load fluctuations of the circuit, and an RF transponder IC card using such a power supply circuit.
2. Description of the Related Art
In general, an electronic circuit such as an analog circuit or a digital circuit is operated by applying a power supply voltage thereto. Fluctuations in a current flowing through the circuit depend on the type of the circuit. In particular, large current fluctuations occur in a digital circuit such as a CPU, a logic circuit, or a memory. Such current fluctuations lead to generation of power supply noise or fluctuations in a power supply voltage. The mechanism of generation of power supply noise will be described below.
FIG. 11 is a diagram illustrating a structure of a conventional semiconductor integrated circuit 3. The semiconductor integrated circuit 3 includes a digital circuit 300 and an analog circuit 310. The digital circuit 300 includes a CPU 301 and a memory 302. The semiconductor integrated circuit 3 is externally supplied with a power supply voltage VDD via an inductor L1. The inductor L1 is a parasitic inductor which occurs in a conductor line or a bonding wire and has a value of, for example, several nanofarads. In this case, an internal power supply voltage of the semiconductor integrated circuit 3 (chip) is VDD1.
In the case where a current I1 flowing through the semiconductor integrated circuit 3 (the digital circuit 300 and the analog circuit 310) varies as illustrated in FIG. 12B, the internal power supply voltage VDD1 of the semiconductor integrated circuit 3 (chip) fluctuates at the time the current I1 varies, as illustrated in FIG. 12A.
Power supply noise generated in such a manner adversely affects a circuit operation of the analog circuit 310. For example, in the case where the analog circuit 310 is a comparator having a hysteresis property, a comparison value varies due to fluctuations in the power supply voltage VDD1, so that malfunction in the analog circuit 310 may be caused. In the case where the analog circuit 310 is an amplifier, there arises a problem that a power supply fluctuation component is added to the power supply voltage VDD1 and the quality of a signal output from the amplifier deteriorates. Specific examples of malfunction and property deterioration of an analog circuit will be given below.
FIG. 13 is a graph illustrating input-output characteristics of a comparator having a hysteresis property. In general, a comparator is in a HIGH state when an input voltage is higher than a reference voltage, and the comparator is in a LOW state when the input voltage is lower than the reference voltage. In contrast, in a comparator having a hysteresis property, as illustrated in FIG. 13, an output signal Vout is in a HIGH state when an input voltage (an input signal Vin) is equal to or higher than a voltage VH which is higher than a reference voltage Vref1 by an offset voltage, and the output signal Vout is in a LOW state when the input voltage (the input signal Vin) is equal to or lower than a voltage VL which is lower than the reference voltage Vref1 by the offset voltage. In the case where an output of the comparator having a hysteresis property is in the HIGH state and a voltage input to the comparator is equal to the reference voltage Vref1, when the input voltage is reduced to be equal to or lower than the voltage VL due to generation of power voltage noise, the output of the comparator is changed from the HIGH state to the LOW state and a value of the output will not return to the previous value (HIGH state).
In order to solve such a problem, conventionally, a method which uses separate power supplies for an analog circuit and a digital circuit or provides a capacitance section having a large capacitance between the power supply voltage VDD1 section and a ground VSS in the semiconductor integrated circuit 3 has been used to restrain fluctuations in a power supply voltage.
However, the above-described methods lead to an increase in the number of pins (terminals) of the semiconductor integrated circuit 3 (chip) or an increase in area of the capacitance section provided in the semiconductor integrated circuit 3 (chip). It is preferable that a value of the capacitance is equal to or more than 1 xcexcF, but a value of capacitance which can be provided in the semiconductor integrated circuit 3 (chip) is about 1 nF at most in the case where a standard CMOS process is used, and this is not very effective for great load fluctuations of the semiconductor integrated circuit 3 (chip).
A power supply circuit of the present invention includes: a current detection circuit connected between a first power supply voltage section for applying a first power supply voltage to a first electronic circuit and a second power supply voltage section for applying a second power supply voltage to a second electronic circuit, the current detection circuit having a monitor terminal for monitoring a current flowing from the first power supply voltage section to the second power supply voltage section; and a current compensation circuit connected to the first power supply voltage section and the monitor terminal, the current compensation circuit controlling a compensation current flowing from the first power supply voltage section to a ground based on the monitored current to compensate for current fluctuations caused by load fluctuations of the second electronic circuit, thereby achieving an objective of the present invention.
According to above-described structure, the current detection circuit is used to monitor the current flowing through the second power supply voltage section to detect current fluctuations caused by load fluctuations of a digital circuit, etc., and the power supply compensation circuit is used to compensate for the current fluctuations to restrain generation of power supply noise and fluctuations in a power supply voltage, thereby realizing a high-performance power supply circuit which generates lower noise and can restrain malfunction of a circuit (e.g., an analog circuit) which is liable to be adversely affected by the generation of power supply noise and fluctuations in a power supply voltage or a deterioration in the quality of a signal output from a circuit (e.g., an analog circuit).
The current detection circuit includes a linear regulator including an operational amplifier having a noninverting input terminal and an output terminal, a transistor, and first and second resistances, the transistor has a source connected to the first power supply voltage section, a gate connected to the output terminal of the operational amplifier, and a drain connected to the second power supply voltage section, the first and second resistances are connected in series between the second power supply voltage section and the ground, and a contact point between the first and second resistances is connected to the noninverting input terminal of the operational amplifier, and the operational amplifier noninverting input terminal is connected to a reference voltage section and the operational amplifier output terminal functions as the monitor terminal.
According to the above-described structure, as described in the following Example 1, it is possible to monitor current fluctuations caused by load fluctuations of a digital circuit, etc., using the linear regulator.
The current detection circuit includes a third resistance connected between the first power supply voltage section and the second power supply voltage section, and a monitor terminal on the second power supply voltage section side.
According to the above-described structure, as described in the following Example 2, it is possible to monitor current fluctuations caused by load fluctuations of a digital circuit, etc., using the current detection circuit.
The current compensation circuit includes: a subtraction circuit for producing a difference current between a prescribed current and a current controlled by the monitor terminal; and a current circuit for causing a compensation current proportional to the difference current to flow from the first power supply voltage section to a ground.
According to the above-described structure, as described in the following Example 1, it is possible to compensate for current fluctuations of a digital circuit, etc., using the power supply compensation circuit.
The current compensation circuit includes: a reference voltage generation circuit for producing a reference voltage; a differential amplifier circuit for producing a current corresponding to the sum of a prescribed current and a difference current, the difference current being proportional to a value of a difference voltage representing a difference between the reference voltage and a voltage at the monitor terminal; and a current circuit for causing a compensation current proportional to the sum of the prescribed current and the difference current to flow from the first power supply voltage section to a ground.
According to the above-described structure, as described in the following Example 2, it is possible to compensate for current fluctuations of a digital circuit, etc., using the power supply compensation circuit.
The first power supply voltage section is connected to an analog circuit.
According to the above-described structure, it is possible to restrain malfunction of an analog circuit which is liable to be adversely affected by fluctuations in a power supply voltage or a deterioration in the quality of a signal output from a circuit.
The second power supply voltage section is connected to a digital circuit.
According to the above-described structure, it is possible to detect current fluctuations caused by load fluctuations of a digital circuit, etc., in which large current fluctuations occur.
The current circuit causes the compensation current to flow via a fourth resistance to a ground.
According to the above-described structure, it is possible to restrain generation of power supply noise by compensating for fluctuations in a current flowing through the second power supply voltage section. The sum of the compensation current and the current flowing through the second power supply voltage section may not be constant. In this case, power supply noise to be generated can be reduced by partially compensating for current fluctuations.
The sum of a current flowing from the first power supply voltage section to the second power supply voltage section and the compensation current flowing from the first power supply voltage section to a ground is constant.
According to the above-described structure, it is possible to restrain a current which is caused to unnecessarily flow through a current circuit due to a variance in element properties, etc., of a transistor provided in the current circuit.
An RF transponder IC card of the present invention includes: a coil antenna; and a semiconductor integrated circuit including a tuning capacitance section having a tuning capacitance, a charging capacitance section having a charging capacitance, a rectifier circuit, an analog circuit, a digital circuit, and a power supply circuit which includes a current detection circuit connected between a first power supply voltage section for applying a first power supply voltage to a first electronic circuit and a second power supply voltage section for applying a second power supply voltage to a second electronic circuit, the current detection circuit having a monitor terminal for monitoring a current flowing from the first power supply voltage section to the second power supply voltage section and a current compensation circuit connected to the first power supply voltage section and the monitor terminal, the current compensation circuit controlling a compensation current flowing from the first power supply voltage section to a ground based on the monitored current to compensate for current fluctuations caused by load fluctuations of the second electronic circuit. The coil antenna is connected in parallel with the tuning capacitance section to an input of the rectifier circuit, the rectifier circuit has an output connected to the charging capacitance section and produces the first power supply voltage supplied to the analog circuit and the power supply circuit produces the second power supply voltage supplied from the first power supply voltage section to the digital circuit, thereby achieving an objective of the present invention.
According to the above-described structure, it is possible to restrain malfunction of an analog circuit, etc., caused by load fluctuations in a digital circuit, etc., or the deterioration in the quality of a signal output from the analog circuit, thereby realizing a high-performance RF transponder IC card.
The analog circuit includes a demodulator circuit.
According to the above-described structure, as described in the following Example 3, it is possible to realize a high-performance RF transponder IC card.
The semiconductor integrated circuit further includes a modulator circuit connected in parallel with the tuning capacitance section to an input of the rectifier circuit.
According to the above-described structure, as described in the following Example 4, it is possible to realize a high-performance RF transponder IC card having a modulator circuit.
The modulator circuit modulates a circuit impedance using a modulating signal.
According to the above-described structure, as described in the following Example 4, it is possible to realize a high-performance RF transponder IC card having a full-wave rectifier.
The rectifier circuit is a full-wave rectifier circuit.
According to the above-described structure, as described in the following Example 4, it is possible to realize a high-performance RF transponder IC card having a full-wave rectifier. A half-wave rectifier may also be used as the rectifier. In such a case, it is possible to realize a high-performance RF transponder IC card having a half-wave rectifier.
Thus, the invention described herein makes possible the advantages of providing: a high-performance power supply circuit and a high-performance RF transponder IC card which can restrain the generation of power supply noise and fluctuations in a power supply voltage by compensating for current fluctuations caused by load fluctuations of a digital circuit or the like and which can restrain the malfunction of a circuit (e.g., an analog circuit) which is liable to be adversely affected by the generation of power supply noise and fluctuations in a power supply voltage or a deterioration in the quality of a signal output from a circuit (e.g., an analog circuit).
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.